In the past, there has been extensive description in the patent and other technical literature of electrographic developers and photoelectrophoretic imaging processes.
Descriptions of photoelectrophoretic imaging processes are contained in U.S. Pat. Nos. 2,758,939 by Sugarman issued Aug. 14, 1956; 2,940,847, 3,100,426, 3,140,175, 3,143,508, 3,384,565, 3,384,488, 3,615,558, 3,384,566, 3,383,993, and 3,976,485. In each of the foregoing photoelectrophoretic imaging processes an imaging layer comprising electrically photosensitive material is subjected to the influence of an electric field and exposed to an image pattern of electromagnetic radiation to which the electrically photosensitive material is sensitive. The electrically photosensitive material is caused to migrate imagewise in the layer to form a record of the imaging electromagnetic radiation.
Electrophotographic processes using electrographic developers are described in U.S. Pat. Nos. 2,221,776, issued Nov. 19, 1940; 2,277,013, issued Mar. 17, 1942; 2,297,691, issued Oct. 6, 1942; 2,357,809, issued Sept. 12, 1944; 2,551,582, issued May 8, 1951; 2,825,814, issued Mar. 4, 1958; 2,833,648, issued May 6, 1958; 3,220,324, issued Nov. 30, 1965; 3,220,831, issued Nov. 30, 1965; 3,220,833, issued Nov. 30, 1965; any many others. Generally these processes have in common the steps of forming a electrostatic charge image on an insulating electrographic element. The electrostatic charge image is then rendered visible by treatment with an electrographic developer.
Generally electrographic developers include a toner which is electrostatically attractable to the charge image. The toner in electrographic imaging is usually a particulate polymeric material containing a colorant such as a pigment for viewing purposes.
In color electrophotographic and color photoelectrophoretic imaging it is important that the pigments used have the proper light absorption characteristics. Thus, a cyan pigment should have a high absorption of red light (about 600-700 nm) and very low absorption of green light (about 500-600 nm). Many cyan pigments such as copper phthalocyanine, do not possess these absorption characteristics to the desired extent.
Making a cyan pigment having the desired absorption characteristics is difficult. Indeed, Andre Pugin states in the Official Digest, Vol. 37, page 782, July, 1965, that there is no known relationship between chemical modification of a crystalline material, such as a pigment, and its color and other properties.